Installation for drying waste, in particular wastewater purifying sludge

ABSTRACT

The system dries pasty or powdery products such as sludge from wastewater purifying stations, the drying occurring in a chamber receiving the products to be dried. The system includes a greenhouse with translucent or transparent walls erected on a slab upon which a bed of products to be dried is deposited. At least one wind generator is provided for generating energy which is converted to heat that is delivered to the slab upon which the bed of products to be dried is spread.

FIELD OF THE INVENTION

The present invention relates to the drying of waste, particularly ofsludge from water purifying treatment, and it proposes to provide adrying device employing two renewable energy sources: solar energy andwind energy.

BACKGROUND OF THE INVENTION

This invention applies in particular, without being limited thereby, tothe drying of biological sludge from urban and/or industrial wastewaterpurifying stations.

The present invention is vitally important at a time when, throughoutthe world, research and applications are being developed on techniqueswhich serve to conserve fossil energy resources and to control globalwarming due to the greenhouse effect.

A person skilled in the art knows that purification sludge is aninevitable waste produced by water treatment: every individual producesan average of 20 kilograms per year thereof, representing more than 10million metric tons per year for Europe, expressed as dry matter,representing a 5 times larger figure when applied to so-called wetsludge, which consists of 20% dry matter and 80% water.

At the present time, this sludge can be disposed of in several ways, inparticular: dumping, incineration and agricultural spreading. However,owing to numerous constraints (in particular technical, health,regulatory) these sludge disposal systems are increasingly complex,hence increasingly expensive, and even liable to criticism and indeedprohibited locally.

At all events, drying the sludge is an unavoidable step in thewastewater treatment system, because it serves to reduce the volumes tobe stored by a factor of 4, facilitating transport and disposal. Thisdrying clearly represents a rapidly growing market today.

The prior art in this field can be summarized as follows:

-   -   Thermal drying: this technique uses conventional energy (gas,        oil, electricity) as well as specific equipment to transfer        energy to the sludge and to evaporate the water present therein.        The application of this technique requires large-scale        installations, incurring large investments and consuming        colossal amounts of energy (1 megawatt per metric ton of water        evaporated) which also generate highly polluted condensates        which must then be retreated at considerable expense.    -   Composting: this natural, long-established method, using        thermophilic fermentation of the carbonaceous organic matter in        the presence of air, causes a heat release that permits partial        dehydration of the water present in the sludge. This is a simple        method, easy to apply, economical, and capable of yielding a        high-grade product. However, it is ineffective for reducing the        volume of the sludge, because the drying is only partial and        requires the addition of structuring co-products (bark, sawdust,        green waste, etc).    -   Solar drying: this technique is starting to be developed in the        field of sludge drying because it is suitable for building        simple, economical and ecological installations. However, it        presents the major drawback of requiring large greenhouse areas,        incurring very large and hence costly investments, as well as        problems of layout, thus being limited to rural areas and        low-capacity stations.

So far, it has not been possible, on an industrial scale, to builddrying installations that make use of wind energy. This is because thegeneration of energy by wind generator to supply a conventional sludgedrying installation is uncompetitive. Furthermore, hybrid solutions (gascompression, heat pumps) have limited capacity and are only feasible forvery small installations. A further drawback of wind energy is that itis not constantly available, therefore requiring oversized dryingequipment (by a factor of at least 2), which would lead to uncompetitivesolutions.

BRIEF DESCRIPTIONS OF THE INVENTION

Based on that prior art, the present invention proposes a solution thatcorrects the above mentioned drawbacks, while permitting theconstruction of competitive sludge drying installations, of limitedarea, consuming no other energy than renewable energies, and permittingan effective reduction of the sludge volume.

This technical problem is solved by a device for drying pasty or powderyproducts such as waste and more particularly sludge from wastewaterpurifying stations, in a chamber receiving the products to be dried, andwhich is provided with means for turning over and routing said products,this device being characterized in that it comprises a translucent ortransparent greenhouse, erected on a slab upon which the bed of productsto be dried is deposited, this slab being equipped with said turningover and routing means, and further comprises one or more windgenerators generating energy which is degraded in the form of heat whichis restored in the slab upon which the bed of products to be dried isspread.

As may be understood, the device according to the invention allowsdrying at the surface and at the bottom portion of the product to bedried, by implementing two drying techniques respectively:

-   -   solar drying, which is obtained by heating the surface of the        bed of products to be dried, by the solar radiation passing        through the transparent or translucent walls of the greenhouse;    -   wind drying: wind generators generate energy for heating the        bottom portion of the products to be dried. This effect can be        obtained:        -   either simply by a wind generator generating electricity            that supplies resistors embedded in the slab supporting the            greenhouse;        -   or in a more complex manner, by using a heat pump which is            actuated by the wind generators, the hot source heating the            slab and the cold source being used to lower the moisture            content of the scavenging air.

Other features and advantages of the present invention will appear fromthe description provided below with reference to the drawings appendedhereto in which:

BRIEF DESCRIPTIONS OF THE FIGURES

FIG. 1 is a schematic perspective view showing an embodiment of aninstallation according to the invention, and

FIG. 2 is an elevation view showing a variant of this installation.

Reference should be made first to FIG. 1. In this figure, it may beobserved that, in this non-limiting embodiment, the drying installationaccording to the invention comprises:

-   -   a greenhouse 1 having walls made of translucent or transparent        plastic, allowing the passage of solar radiation and resting on        a heating slab or floor 2 upon which the bed 3 of products to be        dried is spread, for example biological sludge from water        purifying stations. The products to be dried are disposed in a        thin layer, that is with a thickness of about 20 to 100 cm. As        mentioned below, this floor 2 further comprises means for        turning over and routing the products to be dried, and    -   at least one wind generator 4 generating energy restored in the        form of heat for heating the slab 2.

In the embodiment shown in FIG. 1, the wind generators such as 4generate electricity to supply a battery of electrical resistorsembedded in the slab 2 and capable of dissipating, in this slab, themaximum power supplied by the wind generators.

The installation further comprises ventilation means combining naturalventilation with intermittent forced ventilation 3 e. It also comprisesregulating means comprising sensors measuring the temperature of thesludge 3 to be dried 3 a, the air temperature respectively 3 b insideand outside the greenhouse 1 and the humidity of this air 3 c, thesevarious sensors actuating mobile louvers 5 and 6 which are provided onthe walls of the greenhouse, respectively at the bottom and top portionthereof, in order to regulate the natural convection which removes themoisture-laden air, either naturally or by forced ventilation. Thissystem can be supplemented by programming and automation means 3 b,making it possible, for example, to program a week of operation.

As regards the means which are provided in the heating slab or floor 2,for the turning over and routing of the product to be dried, they can beprovided, for example, in the form of rotating rollers, equipped withtoothed harrows serving to break up the bed of sludge to be dried 3,transporting said sludge from the inlet to the outlet of the greenhouse,while turning it over and aerating it.

The equipment of the installation according to the invention describedabove, which provides, on the one hand, the turning over and routing ofthe product to be dried and, on the other, the forced ventilation, canbe supplied with wind energy whereof they would only use about 10% ofthe capacity normally provided on the installation. In case of “winddeficit”, energy from the electrical grid would be used in case ofperi-urban installation, or photovoltaic panels could be used in aremote rural environment.

Reference should now be made to FIG. 2, which shows a variant of theinstallation according to the invention. This variant comprises the samemeans as those described above with reference to FIG. 1, the onlydifference being due to the fact that it uses a heat pump.

This embodiment serves to make the installation according to theinvention more effective, particularly in windy and humid areas, forexample in oceanic maritime climates.

The wind generators such as 4 actuate a heat pump (7) comprising acompressor and two heat exchangers: one (hot source) transfers heat tothe slab 2 in order to heat the bottom portion of the bed 3 of materialto be dried, the second 8 (cold source) condenses the moisture in theair entering the greenhouse 1, thereby permitting the scavenging of thisair with dry unsaturated air, thereby increasing the efficiency of theinstallation.

An example is given below of the application of an installationaccording to the invention, highlighting the technical advantages andeffects that it procures, in comparison with conventional dryinginstallations.

This installation is used for treating the effluents of a city of 20,000inhabitants. Sludge production is 400 metric tons per year of dry matter(20 kg per capita), or 2,000 metric tons of sludge with a dryness of 20%(hence containing 80% water). This dryness is such as conventionallyobtained by mechanical dehydration. The drying installation according tothe invention comprises a greenhouse having an area of 700 m² and a windgenerator with a rated capacity of 100 kW supplying a battery ofelectrical resistors embedded in the slab 2. This installation issuitable for obtaining sludge with a dryness of 75% or more.

A conventional drying installation for obtaining a dryness comparable tothat obtained by the installation according to the invention wouldpresent the following features, depending on the technique applied:

-   -   thermal drying: in continuous operation this would consume 200        kWh of natural gas, fuel oil or electricity for 8,000 hours per        year.    -   composting: this would require 4,000 metric tons of structuring        co-products (for example sawdust);    -   solar drying: this would require a greenhouse with an area of        1,500 m².

The invention further provides the following supplementary advantages:

-   -   the cost of construction of an installation according to the        invention is about the same as for conventional drying        installations, whether thermal drying or composting;    -   on the contrary, the operating cost is reduced by a factor of 5,        due to the non-use of fossil energy (direct or indirect thermal        drying) or of co-products (composting), and the mechanical        maintenance and manpower costs are furthermore significantly        reduced in the case of the invention;    -   finally, the ground area of an installation according to the        invention can be half that of a solar drying installation, due        to the additional energy provided by wind energy.

It must be clearly understood that this invention is not limited to theembodiments described and/or mentioned above, but includes all thevariants thereof. In particular, the drying installation could comprisea plurality of corridors operating in series or in parallel, as well asa plurality of wind generators supplying heating networks inpredetermined zones. Similarly, as already mentioned above, theinstallations according to the invention can be applied to the drying ofproducts other than purification sludge: for example for wet waste, forpasty or powdery products or even to evaporate solvents other thanwater.

1. A system for drying sludge from wastewater purifying stations, thedrying occurring in a chamber receiving the products to be dried, thesystem comprising: a greenhouse with translucent or transparent wallserected on a slab upon which a bed of products to be dried is deposited;at least one wind generator generating electrical energy which isconverted to heat that is delivered to the slab upon which the bed ofproducts to be dried is spread; and regulating means having sensorsmeasuring the temperature of the bed of products to be dried, the airtemperature respectively inside and outside the greenhouse and thehumidity of the air, these sensors actuating mobile louvers provided,respectively, in the top portion and in the bottom portion, on the wallsof the greenhouse in order to regulate the natural convection removingmoisture laden air.
 2. The installation as claimed in claim 1, furthercomprising programming and automation means.